If you're travelling in the hotter countries and on a budget...
Step 1: Ask!
In hostels in particular before you seal the deal, check if the air conditioning is to be used at your discretion or if it only works after a certain time (like after the sun is gone and you don't really need it) which is what I found to be the case.
Step 2: No Go, or Go Elsewhere
In which case don't shell out the extra for the 'air conditioned' room.